It is well known to produce synthesis gas (which mainly contains carbon monoxide and hydrogen, and in addition unconverted hydrocarbons and steam) by means of reforming e.g. natural gas with steam which may comprise carbon dioxide. The reforming reactions EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 EQU CH.sub.4 +CO.sub.2 .fwdarw.2CO+2H.sub.2
are highly endothermic; substantial amounts of energy are therefore required to sustain a reforming process and to maintain reactor temperatures at adequate levels. The required energy can either be supplied inside the reaction zone (e.g. by continuously or intermittently allowing an exothermic reaction to take place therein, such as the combustion of hydrocarbons with air) or outside said reaction zone by combustion of fuel in burners which are usually located adjacent to the reforming reaction zone inside a reformer furnace.
In most cases the sensible heat left in combustion gas which has been heat exchanged with the reforming reaction zone is employed for generating and superheating steam which in turn can be advantageously employed in subsequent process steps such as hydrogen production, ammonia- and methanol-synthesis.
However, in some cases the steam requirement of processes in which synthesis gas is used as feed and which are carried out near the reforming unit, is considerably less than the amount of steam produced by using the sensible heat of combustion gas which, after heat exchange with the reaction zone, may still have a temperature of 1000.degree. C. or even 1100.degree. C.